Before the discovery of the cosmic microwave background, it seemed possible that we might be living in a "steady-state universe" with overall properties that do not change with time. The steady-state model, like the Big Bang model, assumes an expanding universe, but does not assume a "creation event." Instead, matter is assumed to be created continuously everywhere in space to ensure that the average density of the universe remains constant. Search the World Wide Web for information about the steady-state theory. Explain why the existence of the cosmic microwave background was a fatal blow to the steady- state theory.

The discovery of the cosmic microwave background (CMB) is one of the most significant observations in cosmology. Imagine looking up at the night sky and seeing a faint glow permeating the cosmos. That's the CMB, often described as the afterglow of the Big Bang. Detected uniformly in all directions, this relic radiation provides a snapshot of the universe as it was when it was only about 380,000 years old, a baby compared to its current age of approximately 13.8 billion years.

The CMB has a nearly perfect black-body spectrum and exhibits a very specific pattern of hot and cold spots that reflect the tiny temperature fluctuations in the early universe. These fluctuations are the seeds of all current structure: galaxies, clusters of galaxies, and vast cosmic voids. The CMB is a cornerstone piece of evidence for the Big Bang model since its existence, and properties are best explained by a universe that was once very hot and dense and has since cooled due to expansion over billions of years.

The Big Bang model is the leading explanation about how the universe began. At its simplest, it talks about the universe as we know it starting with a small singularity, then inflating over the next 13.8 billion years to the cosmos that we know today.

The evidence for this model includes the aforementioned cosmic microwave background, the observed large scale structure of the universe, and the amounts of various chemical elements that were created in the first few minutes of the universe's existence. According to this model, the universe began in a state of extremely high temperature and density and has been expanding ever since. This expansion is not of the universe into space, as if space were an existing container, but rather the creation of new space between objects in the universe.

A cosmological model is a mathematical and conceptual framework for understanding the structure, dynamics, and history of the universe. These models are based on scientific theories and observations and use the language of physics and mathematics to describe and predict cosmic phenomena. Beyond the Big Bang and steady-state theories, there are alternative models, though none so compelling in terms of the breadth and variety of evidence explained.

The Big Bang and steady-state models represent two different cosmological theories about the universe's nature and evolution. In recent times, the Big Bang model has been supported by substantial evidence from astronomy and physics, such as the cosmic microwave background, the abundance of light elements, and the observation of galaxy formation and inflation. Cosmological models are tested against these kinds of observations to validate their accuracy and predictive capabilities.

The observation that the universe is expanding is a fundamental characteristic that informs our understanding of cosmology. It was first observed by Edwin Hubble in the 1920s when he noticed that distant galaxies were moving away from our own Milky Way. This movement is not due to galaxies physically traveling through space but rather due to the expansion of space itself.

Over time, the space between galaxies stretches, so they appear to move away from each other. The rate of this expansion is encoded in what we call the Hubble constant. Far from being a static backdrop, the universe is dynamic, with its expansion accelerating due to the mysterious force known as 'dark energy'. This continuous expansion means that the universe's overall scale changes over time, and it has significant implications for the past and future of the cosmos. It corroborates the Big Bang model and is one of the primary reasons the steady-state theory fell out of favor within the scientific community.